This invention relates to oxide dispersion strengthened alloy compositions which can be employed in high temperature services.
A considerable amount of research has been conducted in recent years to develop alloys which can withstand higher and higher temperatures and environments which are increasingly reactive. Such reactive environments include sulfurizing, carburizing, and oxidizing environments, all of which are known to significantly affect plant performance and efficiency for many industrial processes. It is known that the high temperature service properties of iron, nickel, and cobalt based alloys can be substantially improved by dispersion strengthening. Dispersion strengthening involves the uniform dissemination of a large number of discrete sub-micron sized refractory particles throughout the metal matrix. The refractory particles, generally oxides, serve to stabilize the matrix microstructure at elevated temperatures, thereby increasing its tensile strength and stress rupture life at elevated temperatures. Oxide dispersion strengthened alloys which contain aluminum are particularly useful in high temperature applications where reactive environments are encountered because the aluminum reacts with oxygen to form a protective aluminum oxide scale on the surface of the alloy.
Various powder metallurgy techniques are known for preparing such oxide dispersion strengthened alloys which usually include mechanically alloying the oxide particles with the powder metal matrix thereby forming agglomerates in order to achieve a uniform distribution of the oxide particles in the powder matrix. The agglomerates are then usually consolidated and worked to the desired end product. The high temperature mechanical properties of the resulting alloy product are critically dependent on the presence of stable submicron-size inert oxide particles in the matrix. In addition, the high temperature resistance to reactive environments is, to a large degree, dependent on the formation of an aluminum oxide or chromium oxide scale on the surface of the alloy product. The adherence of such oxide scales is generally improved by the presence of the dispersed oxide particles.
The dispersoids of the type employed in the alloys which are of interest herein are those oxide particles having a negative free energy of formation at 1000.degree. C. of at least as great as that of aluminum oxide, in particular yttria. Oxide dispersion strengthened alloys containing oxide particles such as yttria and aluminum which are presently commercially available suffer from serious quality problems. These problems can usually be attributed to a loss of homogeneity of the material because of interaction of aluminum, oxygen, and yttria resulting in the formation of various alumina-yttria mixed oxides. Oxygen is present either during the preparation of the oxide dispersion strengthened alloy or during high temperature service. This interaction results in a coarsening of the yttria particles and depletion of some of the aluminum which would otherwise be available for the formation of a protective aluminum oxide scale on the surface of the alloy product when aluminum is the primary oxide former.
The present invention overcomes these problems by employing one or more alumina-yttria mixed oxides instead of yttria as the dispersoid.